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Full text of "Text Book Of Mechanical Engineering"

N .1

Oscillating-Engine Valve Gear.

An intermediate valve rod F, may, however, be changed from
D to B, Qi^mce versa,) by the reversing lever E, so that F may move
either in the same or the reverse direction of j. When F is at B
the eccentric must lead the crank, as in Fig. 634; but when F is
at D, the eccentric must follow the crank. The intermediate rod
F, again, is only connected to the valve rod G through the lever
LM, the pin K forming a fulcrum upon which LM is rocked by the
crosshead N. The travel, LP, thus obtained, represents twice
(lap + lead), as at hj\ Fig. 640, and takes effect at the dead centre
positions. When F, therefore, is in mid gear at c, the valve opens
only to lead, but when moved to D or B, the opening is eccentric
throw minus lap, as in Fig. 634.

Valve Gear for Oscillating Engines.—The method by
which a satisfactory motion of the valve is obtained will now be
made clear by reference to Fig. 644. T and u are the valve boxes,
of which there are two, in order to keep the cylinder balanced.
Y is the cylinder and v w the trunnions, being steam and exhaust
pipes respectively, supplied with stuffing boxes. M m and N n
are the valve levers, rocking on fulcra R and s; and P Q the valve
spindles, guided at their upper end. All the parts mentioned
share in the rocking motion of the cylinder, the remainder are
either fixed to the snip or take motion only from the crank.
z z are fixed guides for sliding link L, whose slot is curved to a
radius from trunnion centre. To L is again connected, by centre-
pin F, the usual radius link GH, which is moved by eccentrics cd
through rods D E. x is the trunnion bearing.

It will be seen that the rocking of the cylinder can in nowise
affect the vertical movement of the valve levers; but any motion
given by the eccentrics to the link L is faithfully transmitted to
the valve spindles through their levers, the discs j K always lying
in the link L. On account of the introduction of the rocking
levers M and N, the eccentric motion will be reversed. The
eccentrics are therefore set to follow the crank by 90° minus lap
and lead, and the rods are said to be crossed.

Steam enters at v, and passes into the valve chests by the
belts e e, entering through port a. After giving work to the
pistons through either steam port b fr, it exhausts through the
mid port ?, and passes out through the belt f to the exhaust